HVAC-Talk site will be slow for the next few days. It's normal site/server maintenance. Thx -Dad

Welcome to HVAC-Talk.com, a non-DIY site and the ultimate Source for HVAC Information & Knowledge Sharing for the industry professional! Here you can join over 150,000 HVAC Professionals & enthusiasts from around the world discussing all things related to HVAC/R. You are currently viewing as a NON-REGISTERED guest which gives you limited access to view discussions

To gain full access to our forums you must register; for a free account. As a registered Guest you will be able to:

Participate in over 40 different forums and search/browse from nearly 3 million posts.

First off, I believe the house to be comfortable during winter . I set the stat at 68 day and 66 night.

In order to maintain either temp in zero to 10 degree weather, furnace cycles on/off as follows: burner on 3 minutes, fan on 4 minutes, there is a one minute delay from time burner turns on to when fan starts, then fan stays on 2 minutes longer than burner. This cycles 6 times per hour.

In warmer temps, like 20 degrees to 30 degrees, burner is on the same amount of time, but only cycles times per hour.

I have a thermometer on top of thermostat to check if that reading is same as thermometer, it is. The temperature at the stat never varies more than 1/2 degree.

All the rooms in the house i have balanced air flow so for the most part they are same temperature as stat.

This all seems fine to me; however, from reading messages on this board, it would have me believe that my furnace is oversize, uncomfortable and is short cycling?

Is what I describe above short cycling?
The thermostat is a basic model honeywell digital that controls air cond. and heat. It is a single stage 84K BTU furnace and an 1150 SQ single story 50's construction ranch home.

brewyourown, I'm a homeowner as well but your description of furnace cycling is very similar to mine. In my home I have 1714sq ft with 87K BTUH output furnace and its way oversized. I'll let the pro's respond too but sounds like your furnace is oversized as well.

In most cases, the home's construction, insulation values, orientation, etc will dictate the amount of efficient 'set-back settings. The furnace size also has a lot to do with the amount of set-back setting.
The surest way to obtain the best setting would be to take the time to 'record' the actual firing minutes of the burner over a 24 hour period with a set-back.
Then, under the approximate same temperatures, 'record' a 24 hour period WITHOUT any set-back. Compare the actual burner "ON" minutes.
From these burner running times you can decide if a set-back is best or not.
Note; a daily savings of only 10 minutes a day less burner time could amount up to a SAVINGS of 5 HOURS burner running time a month, and would also save the 'wear and tear' on the equipment too.
IMO, a $$$$ substantial savings.

You probably have six cycles per hour because that's the default for fossil fuel heat in Honeywell thermostats. Unless you are in a climate that will go way below zero, it's also somewhat oversized (though maybe not extraordinarily so). I say that based on your time measurements; it sounds like the burner only has to run for about 18 minutes per hour at ambient temps close to zero.

Is this a retail (hardware store) Honeywell, or was it professionally installed? And what efficiency is the furnace?

I got carried away on my previous post and answered your question on the wrong subject ("set-back")
.Sorry about that.

As to your question on whether or not your unit is 'short cycling' , depends on many things.......tightness and R values of the home,size of unit ,btu input/output, duct sizing, etc.
iMO if a proper heat loss/duct design calculations are done and both the unit and ducts are sized accordingly, then even at the lowest design outside temperature the unit should only fire approx 2 - 3 cycles per hour at the most. (burner on for approx 4-6 minutes each time.)
As of this moment my burner comes on 2 times in an hour (6 mins each time)and the temp outside is at 18 F. the home is 1500 sq ft bungalow ,fully heated finished basement, fairly well insulated and tight....74k output.FA oil

Originally posted by deejoe
iMO if a proper heat loss/duct design calculations are done and both the unit and ducts are sized accordingly, then even at the lowest design outside temperature the unit should only fire approx 2 - 3 cycles per hour at the most. (burner on for approx 4-6 minutes each time.)

In my opinion at winter design temperature and a properly done Manual J calculation, the burner should be required at least half of the time, and probably way more than that. Meeting design load with 8-12 minutes of firing per hour is an excellent indication of oversizing, even if in the absence of short-cycling.

I do like 3 cycles per hour, though. A different thermostat (or possibly even a different setting on your existing one) will happily do three cycles per hour. With some furnace oversizing like it sounds you have and 3 cph you may find that the temperature swing between cycles is uncomfortably wide, though.

So yeah, 2-3 cycles per hour, but I would much rather see steady state operation at winter design temperature look more like 15 minutes on, 5 minutes off. Slow and steady makes for every even temperatures, quiet operation, and actually getting the full rated efficiency of the equipment.

It is good to know that we're talking about a hardware store thermostat. I have finally come to accept that consumers can't buy thermostats of the same quality that the pros get through retail channels. The strange thing is that you can often buy an identical looking thermostat of the same brand, but it doesn't have the same configurability (they dumb it down for consumer use, I guess). Also retail thermostats tend to take batteries, which tend to corrode and destroy the unit... and many retail thermostats are just junk all the way across the line (Hunter for one).

I have a semi-finished basement area which has the main trunk line running across it and a large register in it that I keep closed most of the time. I have started using the basement more lately and only open the register when my family or I are down there.

Maybe I should leave this register open half way or thereabouts and keep the basement warm all the time, that would give less air to the upstairs registers and make the furnace cycle longer to satisfy the upstairs stat.

Problem is the basement is really not insulated and is not properly set up to be heated; like i said, it just has an aftermarket register "cut in" to the main trunk line. It does keep that area warm though and is nice to have; even if it is not an ideal heating environment.

With fuel oil costs so high, I'm hesitant to heat this area all the time. Would you think heating it all the time would make the furnace more efficient by creating longer run times and then translate to lower fuel costs even though a larger area is being heated?

I know, very tough question, but i guess I'm just looking for peoples thoughts rather than absolute answers.

A note to wyounger; IF, my burner was to run (as you said the way YOU like it) for 15 minutes, the temperature in the house would rise (override) AT LEAST 3 degrees.
I like the 5-6 minute cycles, its a lot easier on the $$$(wallet.)
Since I experience no temperature swings with only 1 or 2 five minute cycles per hour (even at a 0 F outside temp)I guess the insulation,s R value and the homes' tightness pays off in the long run.

Most units I have worked on lately, including my own, are set up to operate at a couple of 5-6 minute burner cycles an hour with an outside temp of approx 0 to -10 F.and this is all accomplished using a standard Honeywell 3200 magic stat T stat (approx $ 60.)

Do the math; 2- 6 min cycles per hour for a constant 72F over a 24 hour period = 288 minutes per day burner ON time.

Compared to some other homeowners that say their unit cycles 5 times an hour @ 6 minutes each =720 minutes burner ON time.
Quite a difference, eh? ($$$$$)

Deejoe, yep, that's oversized, probably by a factor of two. The operating costs work backwards of how you're thinking.

You're running a 74k output for 288 minutes per day. But burner on-tine is irrelevant without knowing the burner input. For the sake of math, let's assume we're talking about 80% rated efficiency and call it a 92.5k burner input.

What I'm suggesting is that the ideal for your case would be more along the lines of a burner half the size running twice as much. So in this theoretical case, a 46.25kBTU burner input running for 576 minutes a day. By running longer burner cycles, you achieve higher thermal efficiency- because an 80% rating doesn't mean everyone gets exactly 80%. So with the smaller burner running twice as long- thereby using the exact same amount of fuel- you actually get a little more heat output from the same amount of fuel. By outputting heat into the house more gradually, the temperature doesn't rise so rapidly when there is a call for heat. The thermostat can run long burner cycles without roasting you out while still only needing 2-3 cycles per hour.

When you oversize relative to the proper Manual J heat loss, you end up with the system that was upstairs in a house my father owned. You get cold and wish the heat would kick on. You get colder still, and just before you get up to go bump the thermostat because you're cold, the furnace fires up. Hooray! In about a minute, the blower starts, and about a minute later, you feel great. Then a minute after that, you're hot. The burner cycles off, but there's still more heat in the heat exchanger, so the blower stays on until that heat is put into the house. By the time the blower cycles off, you're about to sweat. No matter how cold it got, the furnace burner never ran for more than about four minutes at once, and you wished it could run an even shorter cycle.

Now realistically, I know you said you're oil fired, and there aren't tons of small-input oil fired choices. In an perfect world, though, you could maintain better comfort while using less fuel by working a smaller burner harder, even with an identical nameplate efficiency rating.